III-V compound semiconductor photodiodes are a key component in optical communications. With continued increased demand for higher data rates, faster photodiodes are required. However, there are two fundamental limits to the speed of a photodiode: (i) the transit time, t.sub.r, which is the time to sweep the photogenerated carriers across the p-n junction to be collected by the contact; and (ii) the RC time constant, which is a time associated with the capacitance of the photodiode for the given diode dimensions. Typically the transit time is quite fast, on the order of 5-10 psec, while the RC time constant is usually the limiting factor. To reduce the RC time constant, the photodiode can be made progressively smaller; however, as the photodiode area shrinks, the area available for collecting light diminishes, which in turn significantly reduces the optical signal measured. Since the electrode on the photodiode area shadows a proportionately increasing fraction of the photodiode area, as the device shrinks performance is compromised for smaller photodiodes.
An attempt to reduce the effects of shadowing by the top metal electrode was studied at .lambda.-0.85 .mu.m by using indium tin oxide (ITO) as a transparent top electrode in GaAs based photodiodes. See M. Zirngibl et al. "Characterization of a Top-Illuminated p-i-n Diode With an Indium Oxide Tin Contact", Applied Physics Letters, Vol. 54, No. 21, 22 May 1989, pages 2076-2078.
However, at higher wavelengths the use of ITO as the top electrode is not appropriate because of its absorption losses. For example, for a range of wavelengths ranging from 1 .mu.m to 2 .mu.m the transmission falls off almost linearly from about 87 percent to about 25 percent, respectively. Since attenuation of low loss optical fibers tends to be especially low in the wavelength region between 1.3 and 1.6 microns, there is a need for photodetectors efficient at such wavelengths.